In a mobile communication system, mobile terminals and a wireless base station device communicate with each other through a wireless transmission channel. As communication methods for allowing a wireless base station device to communicate with a plurality of mobile terminals at the same time, FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), and the like have been put to practical use.
In a mobile communication system employing such a communication method, an interference occurring in uplink user signals (hereinafter referred to simply as “user signals”), which are transmitted from mobile terminals to a wireless base station device, is a major issue.
Specifically, a user signal transmitted from one mobile terminal passes through a plurality of different propagation paths due to reflection and diffraction, and is received by the wireless base station device as a plurality of multipath signals that reach with different delay times. The multipath signals interfere with each other, which deteriorates the reception quality of the user signal in the wireless base station device. In view of this, various techniques for suppressing a mutual interference among multipath signals (hereinafter, referred to as “multipath interference”) have been proposed. For example, Patent Literatures 1 and 2 disclose a multipath interference canceller, and Patent Literatures 3 and 4 disclose a technique for suppressing a multipath interference using an adaptive array antenna. In addition, the multipath interference can also be suppressed using an equalizer.
One user signal interferes with another user signal existing at a time and a frequency domain which are identical with or close to those of the one user signal, which leads to deterioration of the reception quality in the wireless base station device. In view of this, various techniques for suppressing an interference caused by another user signal (hereinafter, referred to as “inter-user interference”) have been proposed. For example, Patent Literature 5 discloses an inter-user interference canceller. In addition, the inter-user interference can also be suppressed using the above-mentioned adaptive array antenna. This is because the user signals can be spatially separated from each other so as to prevent the user signals from interfering with each other by controlling the reception directivity of the adaptive array antenna.
Furthermore, a user signal transmitted from a mobile terminal camping on a cell formed by another wireless base station device adjacent to one wireless base station device interferes with a user signal transmitted from a mobile terminal camping on a cell formed by the one wireless base station device. Also, a signal from another mobile communication system or the like interferes with the user signals. The inter-cell interface and the inter-system interface cause deterioration of the reception quality in the wireless base station device. In view of this, various techniques for suppressing the inter-cell interface and the inter-system interface have been proposed. The inter-cell interface and the inter-system interface can be suppressed using the above-mentioned adaptive array antenna, for example.
Hereinafter, a configuration and operation of a typical wireless base station device to which the interference suppression technique is applied will be described with reference to FIG. 5.
A wireless base station receiving device 1x shown in FIG. 5 includes a wireless receiving unit 10 and k (k is an arbitrary natural number) units of interference suppression demodulation blocks 30_1 to 30—k (hereinafter, also collectively denoted by reference numeral 30).
Among these, the wireless receiving unit 10 receives a wireless signal 101 including a plurality of user signal components respectively transmitted from a plurality of mobile terminals (not shown) through an antenna. Not only each user signal component, but also signal components caused by the inter-cell interface and inter-system interface, thermal noise, and the like are superimposed on the wireless signal 101. Multipath signal components corresponding to each user signal component are also present.
The wireless receiving unit 10 outputs a baseband digital signal (hereinafter, referred to as “baseband signal”) 102, which is obtained by performing amplification, frequency conversion from a wireless frequency band to a baseband, quadrature detection, or the like on the wireless signal 101, to the interference suppression demodulation blocks 30_1 to 30—k in parallel. As in the wireless signal 101 described above, each user signal component, multipath signal components corresponding to each user signal component, signal components caused by the inter-cell interface and inter-system interface, thermal noise, and the like are superimposed on the baseband signal 102. The wireless receiving unit 10 typically includes a low noise amplifier, a band-limiting filter, a mixer, a local oscillator, an AGC (Auto Gain Controller), a quadrature detector, a low pass filter, and an A/D (Analog to Digital) converter, and the like.
On the other hand, each of the interference suppression demodulation blocks 30_1 to 30—k includes an extraction timing detection unit 31 and an interference suppression function-equipped demodulation unit 32.
The extraction timing detection unit 31 detects timings for extracting multipath signal components corresponding to one user signal (desired user signal) from the baseband signal 102, and outputs information (hereinafter, referred to as “extraction timing information”) 104 indicating the timing to the interference suppression function-equipped demodulation unit 32. More specifically, the extraction timing detection unit 31 calculates a correlation between the desired user signal and the baseband signal 102 in a scanning manner by using information (a frequency band to be used, spreading code, or the like) unique to a desired user and known symbols (such as pilot symbol), thereby detecting the timings for extracting the multipath signal components.
The interference suppression function-equipped demodulation unit 32 refers to the extraction timing information 104 to extract multipath signal components at each extraction timing from the baseband signal 102. Further, the interference suppression function-equipped demodulation unit 32 estimates a wireless transmission channel for the desired user signal (estimates a phase fluctuation amount depending on wireless transmission paths), and removes a phase fluctuation component from the desired user signal by using the estimation result (for example, performs weighting and combining processing on multipath signal components after aligning the phases of the multipath signal components). The estimation of a wireless transmission channel can be performed using known symbols as described above, for example. Further, the interference suppression function-equipped demodulation unit 32 performs a predetermined interference suppression processing on the desired user signal from which the phase fluctuation component is removed, and outputs a demodulation signal in which various interferences are suppressed (hereinafter, referred to as “interference-suppressed user demodulation signal”). In other words, the interference suppression demodulation blocks 30_1 to 30—k respectively output interference-suppressed user demodulation signals 103_1 to 103—k (hereinafter, also correctively denoted by reference numeral 103) corresponding to user signal components from different mobile terminals.
Thus, the interference suppression processing is performed upon demodulation to thereby improve the quality of the user demodulation signal. This leads to an increase in uplink transmission capacity of the wireless base station device.
Incidentally, recent mobile communication systems employ techniques for controlling a modulation method and a coding method to be applied to user signals for each mobile terminal (for example, AMC (Adaptive Modulation and Coding) in CDMA).
In the mobile communication system employing such control techniques, user signals are more likely to be affected by an interference when a modulation method using a larger number of information bits per symbol is applied or when a coding method using a higher code rate is applied. In this case, the wireless base station receiving device preferably performs the interference suppression processing upon demodulation of user signals.
On the other hand, the user signals are less affected by an interference when a modulation method using a smaller number of information bits per symbol is applied or when a coding method using a lower code rate is applied. For this reason, even if the interference suppression processing is not carried out, high-quality user demodulation signals can be obtained in many cases without causing any problem in operation.
Accordingly, the use of the wireless station receiving device 1x shown in FIG. 5 for the mobile communication systems employing the control techniques described above causes a problem of deterioration in efficiency of user signal demodulation processing including the interference suppression processing. This is because the wireless base station receiving device 1x performs the interference suppression processing on all user signals including user signals in which the effect of the interference suppression processing is hardly obtained. Circuits for executing the interference suppression processing are typically large in size, while the circuit sizes are different depending on the contents of the processing. Additionally, a certain processing time is required to execute the interference suppression processing. In other words, in the wireless base station receiving device 1x, an interference suppression effect large enough to offset an increase in circuit size and processing time cannot be obtained.
For example, Patent Literature 6 discloses an interference suppression method to cope with the above-mentioned problem. In this interference suppression method, whether to perform interference suppression processing upon demodulation of a user signal is determined depending on a modulation method to be applied to the user signal.